It's pretty obvious. The BBFC does not believe that the film warrants an 18 certificate. We probably will see a lot of blood but besides a chestbursting or 2 and the skinned bodies I doubt we'll see anything overly visceral. Context is a huge issue that needs to be taken into consideration. For example a guy smashing someone's head open with a rock would be more offensive than an alien headbiting someone because it's more "real".